^EPHELTXE-KOCKS EX BEAZIL. 



459 



from analogy with other places examined, there should be some- 

 where in the neighbourhood some central mass from which these 

 dykes of phonolite, trachyte, and basalt radiate ; and an examination 

 of the hills in the vicinity will probably reveal such a centre. It is 

 possible, however, that they should be referred to the eruptive mass 

 of the peak of Tingua, which is about 20 miles distant. Here only 

 the lower portion of a single spur has as yet been examined ; 

 this is composed of foyaite similar to that of the principal mass 

 of Cabo Trio, resting upon gneiss. That it has not the character 

 of a dyke in the place examined, is proved conclusively by 

 a tunnel some 400 metres long, which has been cut through the 

 spur from side to side. Gneiss, cut by small dykes of basic rocks, 

 occurs throughout the tunnel and for a few metres above its mouth 

 at both ends, but the surface of the spur above the line of the 

 tunnel is occupied exclusively by foyaite. Small dykes of basalt 

 similar to those of Campo Grande and of a trachytic rock occur ; but 

 thus far they have only been seen in the gneiss. The only 

 phonolite seen is in a large boulder or projecting point in the midst 

 of a foyaite area : it presents the appearance of a dyke about two 

 metres wide, with a sharp line of demarcation between it and the 

 foyaite, which adheres to both sides. The phonolite is thickly 

 spotted with inclusions or segregations of foyaite similar to that of 

 the sides, of all sizes up to an inch or more in diameter. The 

 cross sections of these inclusions show a tendency to geometrical 

 forms, appearing like sections of crystals *. The appearance is that 

 of a dyke which had caught up fragments of the enclosing rock ; but 

 the regularity of the distribution of the inclusions and their similarity 

 of form is against this view, while, on the other hand, the phenomena 

 to be described below from the Caldas locality make it seem 

 plausible to suppose that the phonolite is a portion of the original 

 magma that has escaped complete crystallization, and that the 

 inclusions are crystallized segregations in the midst of it. A. 

 petrographical study will doubtless determine which view is correct. 



The great mountain mass of Itatiaia, rising about 2500 metres 

 above its base, is made up for the most part of a variety of foyaite 

 which has more of the granitic aspect than the prevailing rock at 

 Tingua and Cabo Erio, and which has only been met with in a 

 subordinate mass at the latter place, referred, as already stated, to 

 nepheline-bearing augite-syenite by Prof. Eosenbusch f. Foj'aite 

 of the ordinary type is also known to occur there, as likewise an 

 aphanitic rock, which may be considered a phonolite or a fine- 

 grained foyaite. As the excursion to this peak was made before 

 my attention was drawn to the group of rocks here considered, 

 many other types doubtless passed unnoticed. The neighbouring 



* A similar inclusion of foyaite in the phonolite of Fernando Nbronha was 

 found in a specimen from that place, when no large masses of foyaite were met 

 with in a careful examination of the island. Prof. Eosenbusch, to whom a chip 

 was submitted, regards it as an included fragment of an older rock. 



t A specimen of this rock given to Mr. Henry Bauer, of Iguape, was sent by 

 him to the late Prof. Lasaulx, who described it in a recent number of the 

 ' Sitzungsbericht der niederrheinischen Gesellschaffc.' Bonn. This is the first 

 published notice of the occurrence of these rocks in Brazil. 



